Histamine was first identified as a hormone (G. Barger and H. H. Dale, J. Physiol. (London) 1910, 41:19-59) and has since been demonstrated to play a major role in a variety of physiological processes, including the inflammatory “triple response” via H1 receptors (A. S. F. Ash and H. O. Schild, Br. J. Pharmac. Chemother. 1966, 27:427-439), gastric acid secretion via H2 receptors (J. W. Black et al., Nature 1972, 236:385-390), and neurotransmitter release in the central nervous system via H3 receptors (J.-M. Arrang et al., Nature 1983, 302:832-837) (for review see S. J. Hill et al., Pharmacol. Rev. 1997, 49(3):253-278). All three histamine receptor subtypes have been demonstrated to be members of the superfamily of G protein-coupled receptors (I. Gantz et al., Proc. Natl. Acad. Sci. U.S.A. 1991, 88:429-433; T. W. Lovenberg et al., Mol. Pharmacol. 1999, 55(6):1101-1107; M. Yamashita et al., Proc. Natl. Acad. Sci. U.S.A. 1991, 88:11515-11519). There are, however, additional functions of histamine that have been reported, for which no receptor has been identified. For example, in 1994, Raible et al. demonstrated that histamine and R-α-methylhistamine could activate calcium mobilization in human eosinophils (D. G. Raible et al., Am. J. Respir. Crit. Care Med. 1994, 149:1506-1511). These responses were blocked by the H3-receptor antagonist thioperamide. However, R-α-methylhistamine was significantly less potent than histamine, which was not consistent with the involvement of known H3 receptor subtypes. Therefore, Raible et al. hypothesized the existence of a novel histamine receptor on eosinophils that was non-H1, non-H2, and non-H3. Most recently several groups (T. Oda et al., J. Biol. Chem. 2000, 275(47):36781-36786; C. Liu et al., Mol. Pharmacol. 2001, 59(3):420-426; T. Nguyen et al., Mol. Pharmacol. 2001, 59(3):427-433; Y. Zhu et al., Mol. Pharmacol. 2001, 59(3):434-441; K. L. Morse et al., J. Pharmacol. Exp. Ther. 2001, 296(3):1058-1066) have identified and characterized a fourth histamine receptor subtype, the H4 receptor. This receptor is a 390 amino acid, seven-transmembrane, G protein-coupled receptor with approximately 40% homology to the histamine H3 receptor. In contrast to the H3 receptor, which is primarily located in the brain, the H4 receptor is expressed at greater levels in neutrophils and mast cells, among other cells, as reported by Morse et al. (see above).
Events that elicit the inflammatory response include physical stimulation (including trauma), chemical stimulation, infection, and invasion by a foreign body. The inflammatory response is characterized by pain, increased temperature, redness, swelling, reduced function, or a combination of these. Many conditions, such as allergies, asthma, chronic obstructed pulmonary disease (COPD), atherosclerosis, and autoimmune diseases, including rheumatoid arthritis and lupus, are characterized by excessive or prolonged inflammation. Inhibition of leukocyte recruitment can provide significant therapeutic value. Inflammatory diseases or inflammation-mediated diseases or conditions include, but are not limited to, acute inflammation, allergic inflammation, and chronic inflammation.
Mast cell de-granulation (exocytosis) leads to an inflammatory response that may be initially characterized by a histamine-modulated wheal and flare reaction. A wide variety of immunological (e.g., allergens or antibodies) and non-immunological (e.g., chemical) stimuli may cause the activation, recruitment, and de-granulation of mast cells. Mast cell activation initiates allergic (H1) inflammatory responses, which in turn cause the recruitment of other effector cells that further contribute to the inflammatory response. The histamine H2 receptors modulate gastric acid secretion, and the histamine H3 receptors affect neurotransmitter release in the central nervous system.
Examples of textbooks on the subject of inflammation include J. I. Gallin and R. Snyderman, Inflammation: Basic Principles and Clinical Correlates, 3rd Edition, (Lippincott Williams & Wilkins, Philadelphia, 1999); V. Stvrtinova, J. Jakubovsky and 1. Hulin, “Inflammation and Fever”, Pathophysiology Principles of Diseases (Textbook for Medical Students, Academic Press, 1995); Cecil et al., Textbook Of Medicine, 18th Edition (W.B. Saunders Company, 1988); and Steadmans Medical Dictionary.